The invention relates to a method of measuring movement of a material sheet and a sheet sensor relative to each other along at least one measuring axis, which method comprises the steps of:
illuminating the sheet with a measuring laser beam for each measuring axis, and
converting a selected portion of the measuring beam radiation reflected by the sheet into an electrical signal, which is representative of the movement along said measuring axis.
The invention also relates to a sheet sensor for performing this method and to sheet processing apparatus comprising such a sensor.
Such a method and sheet sensor can be used in an apparatus wherein a material sheet, for example a sheet of paper, is fed through the apparatus in order to undergo processing, for example printing of information or scanning of information printed on a sheet. Such an apparatus may be, for example a (colour) printer, a copier, a document scanner or a facsimile apparatus. A sheet sensor is understood to mean a device by means of which, for example, the velocity of movement of a sheet or the position of a specific area of this sheet can be measured, or the quality of a print can be monitored. In printers, copiers and facsimile apparatus one or more rollers are use to move a paper through the apparatus. Currently, paper movement control, also called paper feed control, in such apparatus is carried out by controlling the movement of a roller. Thereby it is assumed that the movement of the roller exactly determines movement of the paper sheet. However, in practice slippage between the sheet and the roller can not be excluded so that it is not assured the movement of the roller always represents the movement of the sheet. Moreover a fine control of the roller movement, needed for a fine control of the paper feed, requires a very precise control of the motor, which drives the roller. Such a precise control is expensive, which is prohibitive for mass applications. For example, for high-resolution printing such as in a next generation colour printer, the movement of the paper sheet has to be controlled and thus measured with a precision of about 10 xcexcm. Even a very precise mechanical paper feed does not allow such fine control.
The paper feed control could be made more accurate and the requirements for the roller motor control could be lessened considerably by measuring the sheet movement by a beam of optical radiation, which is a contact-less measuring. Devices for optical measurements known so far are intended for small-scale applications and are quite expensive.
It is an object of the invention to provide a method as described in the opening paragraph, which can be carried out with simple means and shows a very high accuracy and liability. This method is characterized in that measuring beam radiation reflected back along the measuring beam and re-entering the laser cavity, which emits the measuring beam, is selected and in that changes in operation of the laser cavity, which are due to interference of the re-entering radiation and the optical wave in the laser cavity and are representative of the movement, are measured.
The new method uses the so-called self-mixing effect in a diode laser. This is the phenomenon that radiation emitted by a diode laser and re-entering the cavity of the diode laser induces a variation in the gain of the laser and thus in the radiation emitted by the laser. The sheet to be measured and the sheet sensor are arranged relative to each other such that the direction of movement has a component in the direction of the laser beam. Upon movement of the sheet relative to the sheet sensor, the radiation reflected and scattered by the sheet gets a frequency different from the frequency of the radiation illuminating the sheet, because of the Doppler effect. Part of the scattered light is focused on the diode laser by the same lens that focuses the illumination beam on the object. Because some of the scattered radiation enters the laser cavity through the laser mirror, interference of light takes place in the laser. This gives rise to fundamental changes in the properties of the laser and the emitted radiation. Parameters, which change due to the self-coupling effect, are the power, the frequency and the line width of the laser radiation and the laser threshold gain. The result of the interference in the laser cavity is a fluctuation of the values of these parameters with a frequency that is equal to the difference of the two radiation frequencies. This difference is proportional to the velocity of the sheet. Thus the velocity of the sheet and, by integrating over time, the displacement of the sheet can be determined by measuring the value of one of said parameters. This method can be carried out with only a few and simple components and does not require accurate alignment of these components.
The use of the self-mixing effect for measuring velocities of objects, or in general solids and fluids, is known per se. By way of example, reference is made to the article: xe2x80x9cSmall laser Doppler velocimeter based on the self-mixing effect in a diode laserxe2x80x9d in Applied Optics, Vol. 27, No. 2, Jan. 15, 1988, pages 379-385, and the article: xe2x80x9cLaser Doppler velocimeter based on the self-mixing effect in a fibre-coupled semiconductor laser: theoryxe2x80x9d in Applied Optics, Vol. 31, No. 8, Jun. 20, 1992, pages 3401-3408. However, up to now, use of the self-mixing effect for measuring the movement of a sheet in a direction at an acute angle with the laser beam has not been suggested. This new application is based on the recognition that a sheet sensor using the self-coupling effect can be made so small and cheap that it can be installed easily and without much additional cost in existing devices and apparatus.
In order to detect the direction of movement, i.e. to detect whether the sheet moves forward or backward along a measuring axis, the method may be characterized in that the direction of movement along said at least one measuring axes is detected by determining the shape of the signal which represents the variation in operation of the laser cavity.
This signal is an asymmetric signal and the asymmetry for a forward movement is different from the asymmetry for a backward movement.
Under circumstances, where it is difficult to determine the asymmetry of the self-mixing signal, preferably another method is used. This method is characterized in that the direction of movement along said at least one measuring axis is determined by supplying the laser cavity with a periodically varying electric current and comparing first and second measuring signals with each other, which first and second measuring signals are generated during alternating first half periods and second half periods, respectively.
The wavelength of the radiation emitted by a diode laser increases, and thus the frequency of this radiation decreases, with increasing temperature, thus with increasing current through the diode laser. A periodically varying current through the diode laser in combination with radiation from the sheet re-entering the laser cavity results in a number of radiation pulses per half period and thus in a corresponding number of pulses in the measured signal. If there is no relative movement of the sheet and the movement sensor, the number of signal pulses is the same in each half period. If the device and the object move relative to each other, the number of pulses in one half period is larger or smaller than the number of pulses in the next half period, depending on the direction of movement. By comparing the signal measured during one half period with the signal measured during the next half period, not only the velocity of the movement but also the direction of the movement can be determined.
This method may be further characterized in that the first and second measuring signals are subtracted from each other.
The changes in the operation of the laser cavity can be determined in several ways.
A first embodiment of the method is characterized in that the impedance of the diode laser cavity is measured.
The impedance of the laser diode is one of the parameters, which change due to the interference effect and is a function of the relative movement of the sheet and the sheet sensor. This impedance can be determined by measuring the voltage across the diode laser and dividing the measured voltage value by the known value of the electric current sent through the diode laser.
A preferred embodiment of the method is characterized in that the intensity of the laser radiation is measured.
Measuring the intensity of the laser radiation is the simplest way of determining the changes in the laser cavity, because this can be done with a simple photo diode.
To increase the accuracy and reliability of the method, this method may be characterized by the further step of detecting the state of focus of the measuring beam relative to the sheet.
The improved method may be further characterized in that the state of focus is detected by determining the amplitude of the changes in the diode laser cavity.
Alternatively, the improved method may be characterized in that the state of focus is detected by imparting a state of focus-dependent shape to a focus detection composed of measuring beam radiation reflected by the sheet and determining this shape by combining output signals of detector elements of a composed radiation-sensitive detector arranged in the path of the focus detection beam.
According to a further aspect of the invention, the method with focus detection can be adapted in a simple way to a method for measuring the thickness of a sheet. The latter method is characterized in that a first optimum state of focus of the measuring beam, relative to the surface of a sheet transporting means, and a second optimum state of focus, relative to the sheet are determined and in that the thickness of the sheet is determined from the difference between the first and second optimum state of focus.
The invention also relates to a sheet sensor for performing measurements on a sheet including the measurement of the movement of a sheet, which sensor comprises at least one laser, having a laser cavity, for generating a measuring beam, optical means for converging the measuring beam to a measuring spot in the plane of the sheet and converting means for converting measuring beam radiation reflected by the sheet into an electrical signal. This sheet sensor is characterized in that the converting means are constituted by the combination of the laser cavity and measuring means for measuring changes in operation of the laser cavity, which are due to interference of reflected measuring beam radiation re-entering the laser cavity and the optical wave in this cavity.
This sheet sensor may be called a self-mixing sheet sensor.
A first embodiment of the sheet sensor is characterized in that the measuring means are means for measuring a variation of the impedance of the laser cavity.
A preferred embodiment of the sheet sensor is characterized in that the measuring means is a radiation detector for measuring radiation emitted by the laser.
The radiation detector may be arranged in such a way that it receives part of the radiation of the measuring beam.
This embodiment of the sheet sensor is, however, preferably characterized in that the radiation detector is arranged at the side of the laser cavity opposite the side where the measuring beam is emitted.
Generally, diode lasers are provided with a monitor diode at their rear side. Usually, such a monitor diode is used to stabilize the intensity of the laser beam emitted at the front side of the diode laser. According to the invention, the monitor diode is used to detect changes in the laser cavity, which are generated by radiation of the measuring beam re-entering the laser cavity.
With respect to the optical design, the most simple embodiment of a sheet sensor for determining the amount and direction of movement of a sheet along a measuring axis is characterized in that it comprises one diode laser and one measuring means for measuring changes in the diode laser cavity and in that the measuring means comprises electronic means for determining said amount and direction.
The electronic means are designed to allow determination of the direction of movement in the ways described herein above.
A sheet sensor for determining the amount and direction of movement of a sheet along a measuring axis with simplified electronic means is, characterized in that it comprises two sensor branches each having a branch axis and including a diode laser and measuring means for measuring changes in the diode laser cavity, the branch axes being arranged in a first plane through the measuring axis and at opposite angles with respect to a normal to the plane of movement of the sheet and situated in the first plane sheet.
As a sheet movement causes opposite effects in the two laser cavities, the amount of movement can be determined by subtracting the detector signals of the two branches from each other, whilst the direction of movement can be determined by comparing the asymmetries of these signals.
An embodiment of the sheet sensor, which allows detection of an oblique movement of a sheet, is characterized in that it comprises two sensor branches, each having a branch axis and including a diode laser and measuring means for measuring changes in the diode laser cavity, the branch axes being shifted in a direction perpendicular to the required direction of movement of the sheet.
By means of these two sensor branches a difference in movement of the left-hand side and the right-hand side of the sheet can be detected. If such difference occurs the sheet movement can be corrected by signals supplied by this embodiment of the sheet sensor An embodiment of the sheet sensor suitable for detecting the state of the measuring beam relative to a sheet to be measured is characterized in that it comprises electronic means for determining the amplitude of changes in the laser cavity, which amplitude represents the state of focus.
In this embodiment of the sheet sensor no additional optical means are needed to measure the state of focus of the measuring beam.
An alternative embodiment of the sheet sensor for detecting the state of focus of the measuring beam relative to the sheet to be measured is characterized in that it comprises a separate, multiple element, detector and optical means for imparting a focus dependent change in the shape of a focus detection beam incident on this detector and composed of measuring beam radiation
In this embodiment of the sheet sensor the state of focus can be measured with simple optical means and without complicated electronics.
The latter two embodiments of the sheet sensor may be further characterized in that it comprises an actuator for setting the position, along the axis of the measuring beam, of a focussing lens arranged in the path of this beam and a focus controller for controlling the actuator in dependency of the detected state of focus.
By means of this measure the focus of the measuring beam relative to the sheet can be kept constant which allows an improved measurement.
The sheet sensor may be adapted such that it allows monitoring of line wise printed ink dots during a print process. This sheet sensor is characterized in that it comprises three sensor branches, each including a diode laser and measuring means for measuring changes in the diode laser cavity, the sensor branches being arranged in series along a line parallel to a printing line.
The sheet sensor may be composed of separate, stand-alone, components so that empty spaces in existing apparatus can be used to accommodate the sensor. Form a manufacturing point of view the sensor is preferably characterized in that it comprises a module having a transparent window and including at least one diode laser and an associated photodiode and a lens arranged between the diode laser and the window, the diode laser being arranged eccentrically with respect to the lens.
Such a module can be made compact so that it can easily build in. The lens may be a rotationally symmetric lens or may have another shape. Due to the eccentric position of the laser with respect to the lens element, it is ensured that the measuring beam is incident on the window of the module at an acute angle so that this beam has a component along the measuring axis. For the following explanation, the term optical axis is introduced, which is understood to mean the symmetry axis of the lens, or the module, which axis is perpendicular to the window of the module.
A sheet sensor suitable for, for example, measuring oblique movement of the sheet or for both measuring a sheet movement and monitoring print quality, is characterized in that it comprises two diode lasers and at least one photo diode.
As will be explained later on, this device and other devices utilizing two or more measuring beams may be provided with a separate detector for each measuring beam. However, it is also possible to use one and the same detector for all measuring beams if time-sharing is used.
In the module a diode laser of the type VCSEL (vertical cavity surface emitting laser) may be used. Such a laser emits radiation in the vertical direction, which makes it suitable for the present module. However, currently such a laser is quite costly, it is not very suitable for consumer mass products.
For this reason, preference is given to an embodiment of the module which is characterized in that each diode laser is a horizontal emitting laser and in that the device comprises, for each diode laser, a reflecting member reflecting the beam from the associated diode laser to the window.
Horizontal emitting diode lasers are the most commonly used lasers and are much cheaper than a VCSEL. Providing the device with a reflecting member adds little to the costs of this device.
An embodiment of the module, which can be manufactured relatively easily and at low cost, is characterized in that it is composed of a base plate on which the at least one diode laser and associated detector are mounted, a cap member fixed to the base plate and comprising the window and accommodating the lens.
This embodiment is composed of only three portions, which can be assembled easily and without severe alignment requirements.
An embodiment of the module, which is even easier to manufacture, is characterized in that the lens is integrated in the cap member having an internal surface which is curved towards the base plate.
This embodiment is composed of only two portions.
These embodiments are preferably further characterized in that the base plate, the cap member and the lens are made of a plastic material.
Components made of such a material may be cheap and low weight and thus are suitable for consumer products. Only the material of the lens should be transparent and have some optical quality.
An alternative embodiment of the module, i.e. a module without a lens, is characterized in that each diode laser is coupled to the entrance side of a separate light guide, the exit side of which is positioned at the window of the sensor.
In this embodiment, the radiation of an illumination beam is well isolated from its surroundings so that cross talk between the movements along different axes is eliminated or strongly reduced.
This embodiment is preferably characterized in that the light guides are optical fibres.
Optical fibres are flexible, have a small cross-section and show little attenuation per length unit and thus allow location of the window of the device at a larger distance from the diode lasers and the detectors.
The invention also relates to an apparatus for processing a material sheet and comprising a sheet transport means and a sheet transport measuring means for controlling the sheet transport. This apparatus is characterized in that the sheet transport measuring means comprises a sheet sensor as described herein above.
By introducing the sheet sensor in an apparatus the performance of this apparatus is improved and the apparatus is distinguished from conventional apparatus. As will be explained in the following the term processing should be interpreted broadly. Embodiments of the apparatus are described in claims 31-34.